1. Field of the Invention
The invention relates to a power supply unit for vehicles, and, in particular, to a power supply unit for vehicles which can easily control electric power supply to a high voltage load.
2. Description of the Related Art
FIG. 6 illustrates a circuit configuration of a conventional power supply unit for vehicles as disclosed in U.S. Pat. No. 4,084,126. In the figure, a generator (ALT) 1 is formed by stator windings 11, field winding 12 to which is supplied a field current, controlled by a field current control circuit 2, from an output terminal A of the generator 1, and rectification circuit 13 which rectifies the AC voltage developed in the stator windings 11 to DC voltage. A change-over relay 4 which contains a movable contact 4a is connected to the output terminal A of the generator 1 to perform switching so as to allow the current to be output from the generator 1 based on the output of a high voltage load controller 3. A terminal H which is connected to one contact 4b of the change-over relay 4 is connected to a high voltage load 5, while a terminal B which is connected to the other fixed contact 4c is connected to a vehicle system voltage load 6 which includes the usual battery 6a. A self-excitation circuit is formed with the voltage of the output terminal A of the generator 1 reaching a value required for supplying field current to the field winding 12.
Next, the operation will be described. After an ignition switch 7 has been turned on, turning on a starter switch (not shown) causes the high voltage load controller 3 to change the mode of the field current control circuit 2 from the no charge (field PTr OFF) mode to the system voltage generation mode at the time the engine (not shown) starts rotating. During the initial stage of voltage generation, the field winding 12 is connected to the fixed contact 4c (terminal B) which serves as the normally contacted contact of the change-over relay 4 as shown in the figure. After field current has been supplied to the field winding by the battery 6a, the generator starts to produce electric power.
In cases where electric power needs to be supplied to the high voltage load 5, when the voltage of the output terminal A of the generator 1 reaches a value which is capable of causing self-excitation, the change-over relay 4 is switched by the high voltage load controller 3 to connect the output terminal A to the terminal H on the high voltage load 5 side. The high voltage load controller 3 further outputs a total self-excitation or high voltage generation mode command to the field current control circuit 2 to put the circuit in the high voltage generation mode. In this way, high voltage electric power is supplied from the generator 1 to the high voltage load 5.
In cases where electric power no longer needs to be supplied to the high voltage load 5, to prevent such problems as reduced life of the change-over relay 4 contacts, dielectric breakdown caused by a surge voltage resulting from load blockage of the generator 1, and deteriorated semiconductor devices high voltage load controller 3 changes the field current control circuit 2 to the no charge mode. Then, after the output current to the high voltage load 5 has been reduced to a certain value or to zero, the controller switches the change-over relay 4 so that it is connected to the terminal B side. This connects the output terminal A of the generator 1 to the vehicle system voltage load 6 side which includes the usual battery 6a. Thereafter, the high voltage load controller 3 puts the field current control circuit 2 into the initial vehicle system voltage generation mode after a time lag of about 1 second. The high voltage load 5 is provided with a fail safe function and a temperature sensor 8 for detecting when power does not need to be supplied. The temperature sensor 8 is connected to a terminal T of the high voltage load controller 3. A noise-eliminating capacitor 9 is provided in the generator 1. A charge lamp 23 is provided between the field current control circuit 2 and the battery a to indicate to the user the various failure modes.
As described above, in the conventional power supply unit for vehicles, electric power is supplied to the high voltage load 5 in the following way. Through the change-over relay 4, electric power is supplied to the field winding 12 of the generator by the battery to cause excitation of the winding. This causes the generator 1 to be initially excited to operate, which puts it in a state allowing electric power generation by self excitation. Then, before a large amount of current produced by the generated voltage is introduced into the vehicle system voltage load 6 which includes the battery 6a, the change-over relay is switched at the proper time so that it is connected to the high voltage load 5 side. However, a complicated timing control is required since the conventional unit is constructed to prevent reduced life of the change-over relay 4 contacts caused by switching of the change-over relay when a large amount of current flows and dielectric breakdown caused by a high voltage surge resulting from the cutting off of a load.